The present invention relates to an apparatus for transferring a liquid having a liquid level sensing function, and more particularly relates to an apparatus for transferring a liquid suitable for applying a clinical automatic analyzer for analyzing body fluid samples.
In an automatic analyzer for automatically analyzing biological samples, as a pre-treatment of analysis there is a process in which a sample and a reagent are pipetted and reacted. Further, the reacted reaction solution sometimes needs to be pipetted. It is a necessary condition for accurately performing the measurement of the sample to accurately perform the pipetting work.
In pipetting the sample and the reagent, a pipetting probe is dipped in the sample or the reagent to suck the sample or the reagent into the pipetting probe using a sucking means such as a syringe. Then, the probe having pipetted the sample or the reagent is moved down inside a reaction container to discharge the sample or the reagent in the probe into the reaction container by a discharging operation of the syringe. Pure water is generally used as a medium for transmitting the operation of the syringe to the probe, that is, as an in-flow-passage moving fluid.
The depth of dipping the probe into the sample or the reagent should be minimized. The reason is as follows. Since the sample or the reagent is attached onto the outer surface of the probe when the probe is dipped into the sample or the reagent, the sample or the reagent attached onto the surface must be cleaned. However, if the probe is inserted into the sample or the reagent more deeply than a necessary depth, the outer wall of the probe is not sufficiently cleaned and the remaining fluid attached on the probe may become a source of contamination. Further, a large amount of water cleaning the outer wall of the probe is necessary if the probe is inserted into the sample or the reagent more deeply than a necessary depth, which is not preferable from the viewpoint of economy.
In order to minimize the depth of the probe inserted into the sample or the reagent, it is inevitable to provide a function for detecting that the probe reaches a level of necessary minimum depth in the sample or the reagent, that is, a level detecting function. For this purpose, a liquid level sensor of electrostatic capacitance type disclosed, for example, in Japanese Patent Application Laid-Open No.1-178826 is used. The detection of a liquid surface utilizes the fact that the body fluid sample and the reagent are conductive fluids.
In the liquid level sensor of Japanese Patent Application Laid-Open No.1-178826, the inside of the vertically movable probe is filled with pure water as an in-flow-passage movable fluid, and the sample is sucked into the probe by operation of the syringe. A sample container is placed on a metallic container holder. The probe formed of a single tube is used as a one electrode for detecting a liquid surface, and the container holder is used as the other electrode. The probe serves as the electrode for detecting liquid surface, and the container holder is grounded. An electrostatic capacitance between the electrodes is converted to an electric signal corresponding to the electrostatic capacitance using a converting circuit. Since the electrostatic capacitance between the electrodes changes before the probe is in contact with a liquid surface of the sample and after the probe is in contact with the liquid surface, a detection circuit detects that the probe reaches the liquid surface when the capacitance changes, and the downward movement of the probe is stopped.
A liquid level sensor using a pipetting probe having two tubes is disclosed in Japanese Patent Application Laid-Open No.7-43369. An inner tube is used as an electrode for detecting liquid surface, and an outer tube is used as an electric conductive shield member. The outer tube is grounded. The end of the inner tube is positioned at a level lower than the end of the outer tube so that the inner tube may be brought in contact with a sample to be pipetted. An electric insulator insulates between the inner tube and the outer tube.
In both of the above-mentioned liquid level sensors disclosed in Japanese Patent Application Laid-Open No.1-178826 and in Japanese Patent Application Laid-Open No.7-43369, the inside of the tube used as the electrode for detecting liquid surface is filled with the in-flow-passage movable fluid which is moved by operation of the syringe of pipetting pump. Even if pure water not having electric conductivity is used as the movable fluid, the electric conductivity of the movable fluid is increased due to the breeding of microbes and the whole flow passage becomes electrically conductive when the movable fluid is used for a long period. Thereby, the electrostatic capacitance between the pipetting probe and the container holder becomes large.
Therefore, the electrostatic capacitance between the pipetting probe and the container holder changes as the probe is moved upward or downward. Such change disturbs the measurement of detecting a liquid surface of a liquid to be pipetted.
On the other hand, in a case of analyzing a trace quantity of ingredient in a sample, occurrence of carry-over between samples is a problem. It is known to use disposable nozzle tips in order to avoid this problem. In a case of using disposable nozzle tips made of an electrically conductive plastic, since the electric conductivity is smaller than that in a case where a straight nozzle of non-disposable type made of stainless steel is used as the probe, the liquid level sensing becomes sensitive to the effect of an increase in electric conductivity of the movable fluid.